A Christian group has launched a battle "for free speech" against London's Mayor, Boris Johnson, after he banned its adverts claiming gay people can be "cured" from buses.

Mr Johnson said the ads by the Core Issues Trust could offend gay people and spark retaliation against the wider Christian community.

The posters said: "Not Gay. Ex-Gay, Post-Gay and Proud. Get over it!" and were in response to a previous poster campaign by Stonewall, the gay rights group, which said: "Some people are gay. Get over it!"

The group is challenging the decision by the Mayor, who is in charge of Transport for London (TfL) in the High Court.

Paul Diamond, appearing for the Core Issues Trust, told the judge: "We believe this is a very important free speech case on whether a totally temperate, restrained advertisement can be put on the sides of London buses."

Image Caption:As Mayor, Boris Johnson has the say on adverts on public transport

Ahead of the hearing, Trust Director Dr Mike Davidson said: "This is all about being free to talk about these issues. It was a mistake to assume these views we were expressing came from entrenched homophobia, and failed to recognise that people who want to walk away from their homosexual feelings are a group in their own right."

The trust will draw attention to a poster which some Christians found offensive. Funded by Richard Dawkins, the academic, and the British Humanist Association in 2009, it said: "There's probably no God. Now stop worrying. And enjoy your life."

Andrea Williams, Director of the Christian Legal Centre, which is supporting the case, said: "In a truly democratic society people should be free to choose their behaviour and move away from homosexual behaviour if they wish.

"The possibility that a section of the public will take offence to a particular viewpoint is not a legitimate ground for restricting freedom of expression."

She said: "Boris Johnson needs to realise his mistake and ensure there is freedom for all in the marketplace of ideas. He cannot prefer one group over another."

A Transport for London spokesperson said: "We intend to oppose the application for a Judicial Review of our decision not to run this advertisement on London buses.

"The advertisement breached TfL's Advertising Policy as in our view it contained a publicly controversial message and was likely to cause widespread offence to members of the public.

"This view was borne out by the high number of complaints we received about the ad and the large number of negative comments on social media and newspaper websites."